Jotun Ballast Tank Treatment
Ship's ballast tanks present special problems in the case of corrosion control. Being subject to intermittent wet and dry cycles of aerated seawater places extreme demands on methods of corrosion control. Harsh service environments are coupled with complex tank structure and difficult accessibility. This makes ballast tanks one of the most costly areas of a vessel for application of the new build in g paint system, and also for later maintenance.
However, at the newbuilding stage we are in a position where the steel has not yet been exposed to the corrosive, salty environment and much of the pretreatment has already been done. It is at this time that an extra investment in extra grinding of edges and extra stripe coating - and perhaps a higher dry film thickness of the paint system - would be of the greatest value to the lifetime of the corrosion protection, and consequently to the lifetime of the vessel.
Renewal of steel and blast-cleaning and application of a new paint system is a very costly procedure.
Jotun Protective Coatings and Jotun Cathodic Protection a.s. together are presenting a solution to shipowners.
The following is a short presentation of this method: Magnesium strips are placed in the empty ballast tank (welded or clamped to the steel). The tank is filled up with salt water for 10 - 15 days. A very strong current is then produced in the tank, leading to the build-up of hydrogen and a soft calciferous layer on the steel surface. This will force off the rust scale which will then fall off and accumulate on the bottom of the tank.
When the tank is emptied, it must be washed down with fresh water if a coating is to be applied. The rust scale must then be removed and the tank dried.
The tank will now have a dry surface, free from scale and from salt. This surface will accept a surface-tolerant mastic coating such as e.g. BALLOXY HB LIGHT. Jotun recommends the application of BALLOXY HB LIGHT in two 200- micron coats, possibly in combination with anodes (in case there are spots which do not have 100% protection by the coating).
The cost of this system has been calculated to be cheaper. The expected lifetime is 5 - 8 years.
After descaling, it is also possible to protect the tank by anodes only. This would of course only be effective when the tank is filled with salt water.
In such a case the cost would be about the same as for the coated tank - with a designed lifetime of four years for the anodes. The concept of pretreatment by use of magnesium strips has been presented to shipowners in Norway.
This presentation has been very well received by the shipowners and the shipbuilders.
